29 replies to this topic

[fallah]

Ankur,

Agreed with the point that control valves are not considered to be suitable for depressurization. The threads such as what is available in below link included the reasons behind this important point.

http://www.cheresour...e-valve-as-bdv/

Fallah

[kkala]

fallah:

1. Web reference of above post helps understand adopted principle, also valid for posts No 20, 25, 26 by ankur2061: emergency depressurization is realized through a ball valve and a downstream orifice. However in Aspropirgos Refinery (Greece) this action is realized through a METSO depressurizing valve with no downstream orifice. Concept by kkala was based on this, judged to be a more developed option, equivalent to an orifice area changing with depressurization time. General information of such depressurizing systems can be seen at http://www.metso.co...ntelligent.pdf, herebelow called RefM. One system was installed at the top of existing deethanizer, another on C3/C4 splitter. Both systems cover fire case, discharging their content (essentially LPG) to flare header. PSVs (sized for fire) have remained. Collected data for e.g. deethanizer depressurization system is in the proceeding paragraph.

2.1 The METSO valve is motor operated through a PLC. In manual mode, Operator adjusts valve opening from Control room, where valve opening (% open) is seen (plus other data). In automatic mode, valve gradually opens under fire (mass flow goes lower and lower) to reduce operating pressure from ~22 barg to ~6 barg in 15 min.

2.2 Valve required flow and upstream pressure in automatic mode were estimated through simulation. Input heat flux per API 521 and autorefrigeration were taken into account. In last minutes of simulation the column contained no liquid (case of dry gas expansion) but pressure had been low enough to avoid BLEVE. The valve seems to satisfy simulation requirements fairly well; excessive velocities are not created in the deethanizer column.

2.3 The METSO valve (tight shut off, class 5) is modulating (not on/off), resembles a "screw", probably being of rotary type (RefM) or similar (not globe or ball). Upstream pipe 8", downstream 14" to flare header. Valve and short pipe downstream are heat traced, no orifice.

3.1 Purpose of depressurization can be seen in http://www.scandpow...191-203082.pdf, para 4.1. There are Emergency depressurizing systems and Operational ones, according to RefM, 1st page. Emergency ones are as above; Operational ones are of smaller size,assumed like the one having started present thread (post No 1), able to restore smaller disturbances; e.g. they can cause autorefrigeration cooling (posts No 6, 10, 11), or reduce an LPG leak by lowering its storage temperature (hence pressure).

3.2 A globe valve in lieu of METSO (no downstream orifice) could have a more or less accepted response to emergency depressurization, but leakage and maintenance issues would be raised in the course of time.
An operational depressurizing valve should be of METSO type too; but it seems that some tolerance exists in practice, e.g., for mentioned PCV of post No 1.

3.3 Mentioned PCV of post No 1 has to be excluded from emergency depressurizing (e.g. to occur during fire) because its size (or size of upstream / downstream piping) is not expected to be as big as needed. However it can be used for operational depressurizing (see 3.1). It is true that switching the PCV to manual mode would be more convenient for this operation. Even by resetting the PCV set point operational depressurization is possible, yet the PCV was not designed for this resetting action.

3.4 Consequently purposes of mentioned PCV are described in posts No 6, 8, 10, 11, 12 (set point above max operating - lower than design pressure). I think additional purposes concern operational depressurization (in manual mode), including autorefrigeration cooling.

[kkala]

Below are comments on posts No 20, 22, 25 by ankur2061, concerning the topic of LPG sphere depressurization. Response has been given by kkala in the post No 27 (Apr 15th, 2012), but here some specific details will be pointed out for clarity. Readers are kindly requested to study post No 27, before reading the comments of present post.
Post No 19, by kkala (for info).
Reading this interesting thread, I am thinking of another use for the PCV. Its set pressure is assumed to be in the DCS (not locally), so it can be manually readjusted. So PCV could be also used for depressurizing during an emergency (e.g. fire case).

Post No 20, by ankur2061 (Apr 7th, 2012).
No it cannot be used for depressurizing until it is sized for depressurization based on API STD 521 guidelines which requires that the vessel pressure be reduced to 50% of the design pressure from its maximum operating pressure in a time of 15 minutes.
Expected size of valve and connected piping does not allow emergency depressurization (e.g. for fire case); besides a globe depressurization valve would function with problems in the long term. These refer to the option without downstream orifice, as applied to Aspropirgos Refinery. Operational depressurization is judged possible and within valve functions, much more convenient in manual mode (post No 27).
You cannot arbitrarily assign another duty to a PCV designed for something else.
The task is to find the duties of mentioned PCV (post No 1).
This is where hardcore design experience comes into picture.
Unjustified hint.

Post No 22, by ankur2061 (Apr 7th, 2012).
Good luck in trying to depressurize an LPG sphere by manually adjusting the PCV set pressure without knowing to what extent the pressure will reduce and whether it is good enough to protect the vessel.
Thanks for the "good luck" wish. Depressurization already covered above.
In fact control valves are not even preferred for emergency depressurization. It is preferred to use on-off valves with downstream restriction orifice (RO) for controlled depressuring to the levels as prescribed in API STD 521.
A more advanced option, equivalent to variable area orifice, is depressurizing valve (not on /off) and no orifice. Such a system has been locally installed. See post No 27 for details and remarks.

Post No 25, by ankur2061(Apr 9th, 2012).
kkala, The OP's original query was related to the suitability and / or function of a PCV on a LPG sphere. There was no mention about any depressurization by the OP. Using the PCV for depressurization during a fire was raised by you.
Query asked for PCV duties, I thought of emergency depressurization. It deems unsuitable, mainly due to small size (post No 27).
Post # 22 & 23 provided a response that control valves are not considered to be suitable for depressurization. Instead of accepting that graciously, you are now trying to divert the topic to whether depressurizing a sphere during a fire is feasible or not.
Both parties had incomplete picture. I do not think I have been dogmatic.
No intent to divert the topic. Emergency depressurization of LPG spheres are (still) considered not feasible in local refineries and many other ones. If so, mentioned PCV is excluded from such a duty (post 24), no search for it. At that time I was not aware of the big size of the connected piping (post 27, 2.3).
The question that a LPG sphere requires depresurization or not becomes a separate topic in itself and should be raised as such. Don't you think so?
The question is whether depressurization can be a duty of mentioned valve. This is within present thread. Varying views have to be handled where they have started. For instance see post No 11 and on, http://www.cheresou...ief-scenarios/ .

Edited by kkala, 16 April 2012 - 02:54 PM.

[kkala]

Previous posts indicate that depressurization can be properly realized by installing:
1. either a ball (on/off) valve and an orifice downstream
2. or a specialized remote operated valve (like METSO, working in manual or automatic mode).
Above concerns emergency depressurization; operational depressurization (post No 27) can use a valve, such as the PCV mentioned in post No 1 or similar.
I believe this is a useful summary from posts No 19 - 28, where the reader can go for details or supporting evidence.

[Zubair Exclaim]

Below are comments on posts No 20, 22, 25 by ankur2061, concerning the topic of LPG sphere depressurization. Response has been given by kkala in the post No 27 (Apr 15th, 2012), but here some specific details will be pointed out for clarity. Readers are kindly requested to study post No 27, before reading the comments of present post.
Post No 19, by kkala (for info).
Reading this interesting thread, I am thinking of another use for the PCV. Its set pressure is assumed to be in the DCS (not locally), so it can be manually readjusted. So PCV could be also used for depressurizing during an emergency (e.g. fire case).

Post No 20, by ankur2061 (Apr 7th, 2012).
No it cannot be used for depressurizing until it is sized for depressurization based on API STD 521 guidelines which requires that the vessel pressure be reduced to 50% of the design pressure from its maximum operating pressure in a time of 15 minutes.
Expected size of valve and connected piping does not allow emergency depressurization (e.g. for fire case); besides a globe depressurization valve would function with problems in the long term. These refer to the option without downstream orifice, as applied to Aspropirgos Refinery. Operational depressurization is judged possible and within valve functions, much more convenient in manual mode (post No 27).
 

 

 

i am confused about re d text above , its contradictory .

 

I also agree PCV under discussion is a normal control valve and should not be used for emergency depressurization, METSO valve is probably a modified form of dpressurization valve with all the necessary SIL level, Clas rating etc .. you cannot compare it with PCV underr discussion

Edited by Exclamation, 07 July 2013 - 12:00 PM.